Pentagonal planar

Figure 16.2 Structure of (a) the anion [Te(S2COEt)3]", the first authentic example of 5-coordinate pentagonal planar geometiy (b) [MeTe(I) S2CNEt2 2] and (c) [PhTe S2CNEt2 2 S2P(OEt)2 ] (see text).

Pentagonal planar ion with two nonbonding electron pairs above and below the plane of the pentagon. 

IS the first example of a pentagonal planar inorganic ion (Fig. I7.3g)., It can be rationalized in terms of five bonding pairs to fiuorine atoms in a plane with a lone pair above and below the plane. The lone pairs appear to be "locked in to the axial positions as the molecule is rot fluxiona). as is the isoelectronic XcF6. 

Figure 6-13. The templated formation of a hydrazone macrocycle. This ligand contains five nitrogen donor atoms and was designed to investigate the properties of metal complexes containing a pentagonal planar donor set. In practice, the complexes of these ligands usually acquire one or two axial ligands to give six- or seven-co-ordinate complexes.

Macrocyclic hypervalent iodine trimer 145 was prepared directly from the oxidation of amino acid 143 [Eq. (117)] [226] self-assembly of the monomeric A3-iodane 144 directed by secondary bonding between iodine and oxygen atom of the amino acid fragment is responsible for the formation of the trimer 145, in which iodine atoms have the pentagonal planar geometry. 

Furan, thiophene, and pyrrole, the three heterocycles parent to all the others of this class, have a pentagonal planar structure. Bond lengths and angles are quantities of relevance to our discussion. The values reported below have been chosen because they are more comparable having been determined by the same authors11-13 using the same procedure. In the valence bond description, these molecules... 

Compounds that appear to contain the [XeF]+ (1) and bent [Xe2F3]+ ions are known although the former is always strongly coordinated to a counterions such as SbF-6. Complex anions include XeF-5, XeF-7 and XeF2-g, the first of which has a unique pentagonal planar structure (3), as expected from VSEPR. 

Pentagonal planar coordination, as in [Te(S2COEt)3] , where two ligands are bidentate and one monodentate, is very unusual. It seems to be due to the presence of two stereochemically active lone pairs. 

The XeF5 anion has an unusual pentagonal planar structure that is much more rigid than the planar equatorial fluorines in the fluxional IF7 molecule. The rigidity is attributed to increased repulsions from the Xe electron pairs.16... 

Consistent with the original supposition that sapphyrins should be good ligands for large cations, Woodward predicted that sapphyrins might form particularly stable chelate complexes with uranyl cation (U02 ), a species that is known to favor pentagonal planar coordination environments. " Unfortunately,... 

Perhaps the only known example of pentagonal planar symmetry is the ion XePs. On the basis of the symmetry of this ion, predict the number of infrared-active xenon-fluorine stretching vibrations. 

E8.11 Using VSEPR theory we can determine that XeFs" has a pentagonal planar molecular geometry, so all five of the F atoms are magnetically equivalent and thus the molecule shows a single F resonance. Approximately 25% of the Xe is present as Xe, which has I = 1/2, and in this case the resonance is split into a doublet. The final result is a composite two lines each of 12.5% intensity from the F coupled to the Xe, and one line of 75% intensity for the remainder. 

The only simple examples are the pentagonal bipyramidal IF7 (see Fig. 1) and the ion j which is pentagonal planar, having two lone-pairs occupying the axial positions (for XeF6 see below). 

Figure 7 The structure of the [TeCEtxan),] anion, showing the pentagonal-planar co-ordination about the tellurium atom (Reproduced from J.CS. Chem. Comm., 1975,408)...

The parent shape is a pentagonal bipyramid (Figure 1.29) with the two lone pairs opposite to each other to minimize lone pair-lone pair repulsions. The [XeF5] anion is therefore pentagonal planar ... 

When structures are determined by diffraction methods, atom positions are located. Thus, in terms of a structural descriptor XeF2 is linear and [XeF5] is pentagonal planar. In the diagrams above we show two representations of each species, one with the lone pairs to emphasize the origin of the prediction from the VSEPR model. 

Another example is given by the XeF " ion, which has the three possible structures shown in Fig. 1.21. The results of vibrational analysis for each are summarized in Table 1.12. It is seen that the numbers of IR-active vibrations are 5,6, and 3 and those of Raman-active vibrations are 6, 9, and 3, respectively, for the D3 , C41 , and D5/ structures. As discussed in Sec. 2.7.3, the XeF ion exhibits three IR bands (550-400, 290, and 274 cm ) and three Raman bands (502, 423, and 377 cm ). Thus, a pentagonal planar structure is preferable to the other two structures. The somewhat unusual structures thus obtained for XeF4 and XeF5 can be rationalized by the use of the valence shell electron-pair repulsion (VSEPR) theory (Sec. 2.6.3). 

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